1. Field of the Invention
The present invention relates to micro filtration membranes and ultrafiltration membranes used for water treatment, such as drinking water production, water purification, and effluent treatment. The present invention also relates to porous membrane modules and to water separation apparatuses including such a porous membrane. Furthermore, the present invention relates to battery separators, charged membranes, fuel cells and blood purification membranes using a porous membrane.
2. Description of the Related Art
Porous membranes have been used in various fields including water treatment, such as water purification and effluent treatment; medical application, such as blood purification; food engineering; battery separators; charged membranes; and fuel cells. In the field of producing drinking water, that is, in use for water treatment, such as water purification and effluent treatment, separation membranes are being substituted for conventional sand filtration and coagulation sedimentation, and are being used to improve the quality of treated water. A large amount of water is treated in these fields. Accordingly, a separation membrane having an excellent water permeability is advantageously used in view of membrane replacement costs and the footprints of apparatuses because an excellent water permeability can lead to a reduced area of the membrane, consequently reducing the size of the apparatuses and saving equipment expenses. The separation membranes are also required to have chemical resistance. In water purification, in order to sterilize permeate and prevent biofouling of membranes, an antiseptic, such as sodium hypochlorite, is added or the membranes are washed with an acid, an alkali, chlorine, a surfactant, or the like. Accordingly, separation membranes using a chemical-resistant material, such as a polyethylene resin, a polypropylene resin, or a polyvinylidene fluoride resin, have recently been developed and put into use. In the water purification field, accidents have surfaced since 1990s in which chlorine-resistant pathogens, such as cryptosporidium derived from livestock excrements or the like, are not completely disposed of in a filtration plant and, thus, contained in treated water. In order to prevent such an accident, separation membranes are required to have high physical strength and sufficient separation properties to prevent raw water from contaminating treated water.
In medical application, porous membranes are being used for blood purification, hemodialysis particularly serving as a substitution for kidney functions, blood filtration and blood filtration dialysis, and removal of waste products in blood by extracorporeal circulation. In food industry, porous membranes are used, in some cases, to separate and remove yeast used for fermentation and liquid condensation. In the field of batteries, porous membranes are being used for battery separators that allow electrolytes, but not cell reaction products, to permeate therethrough. Also, in the field of fuel cells, some porous membranes are used as the base material of macromolecular solid electrolytes. On the other hand, in the ultrapure water production, charged porous membranes are used to enhance ion exclusion characteristics and the purity of produced water, in some cases.
These porous membranes are required to have excellent separation characteristics, high chemical and physical strengths, and an excellent permeability, which shows how much untreated liquid permeates through the membranes.
European Patent Application No. 0037836 has disclosed a wet solution method for forming an asymmetrical porous structure by nonsolvent induced phase separation. In this method, a polymer solution prepared by dissolving a polyvinylidene fluoride resin in a good solvent is extruded from an extrusion head at a temperature much lower than the melting point of the polyvinylidene fluoride resin or is cast on a glass plate for forming. The product is brought into contact with a liquid containing a nonsolvent for polyvinylidene fluoride resins. In the wet solution method, unfortunately, it is difficult to perform uniform phase separation in the thickness direction and the resulting asymmetrical membrane has macro voids. Therefore, the mechanical strength of the membrane is unsatisfactory. Also, many factors of membrane forming conditions influence the structure and characteristics of resulting membranes. It is, therefore, difficult to control the step of forming membranes and reproducibility is poor. U.S. Pat. No. 5,022,990 has relatively recently disclosed a melt-extraction method for forming a porous structure. In this method, a polyvinylidene fluoride resin is melt-kneaded with inorganic particles and an organic liquid. The mixture is extruded from an extrusion head at a temperature higher than or equal to the melting point of the polyvinylidene fluoride resin or is pressed with a pressing machine, for forming. After cooling, the organic liquid and the inorganic particles are extracted. Thus, the porous structure is formed. This melt-extraction facilitates the control of void characteristics and helps prepare relatively uniform, strong membranes without forming macro voids. However, if the inorganic particles are not dispersed well, a defect, such as a pin hole, can occur. Also, the melt-extraction undesirably increases manufacturing costs extremely.
Other techniques for manufacturing a porous membrane have been disclosed in which polyolefin resins, such as polyethylene and polypropylene, are used as raw materials. For example, a polyolefin film containing an inorganic filler is drawn in at least one direction so that interface separation occurs between the inorganic filler and the polyolefin to form voids in the film (for example, Japanese Unexamined Patent Application Publication Nos. 7-26076 and 9-25372). In this technique, however, since the inorganic filler must be extracted to be removed, manufacturing costs increase undesirably. Furthermore, in this technique, it is difficult to control the pore size in membrane surfaces and, therefore, only membranes having a relatively large pore size of 0.1 to 1.0 μm are manufactured.
European Patent Application No. 0245863 illustrates a composite membrane including an ultrafiltration membrane disposed on a porous membrane. In the preparation of this composite membrane, the porous membrane, acting as the base material, is treated with an alcohol solution of glycerin to enhance the affinity for the ultrafiltration membrane. After drying, a polymer solution is applied to the base material and is solidified to form the ultrafiltration membrane. This technique therefore makes manufacturing processes complicated and extremely increases manufacturing costs.